17 Thou shalt not uncover the nakedness of a woman and her daughter, neither shalt thou take her son’s daughter, or her daughter’s daughter, to uncover her nakedness; for they are her near kinswomen: it is wickedness. 

18 Neither shalt thou take a wife to her sister, to vex her, to uncover her nakedness, beside the other in her life time. 

19 Also thou shalt not approach unto a woman to uncover her nakedness, as long as she is put apart for her uncleanness. 

20 Moreover thou shalt not lie carnally with thy neighbour’s wife, to defile thyself with her.

We have been discussing relationships that were too close to permit sexual relations. Today continues that pattern, but extending beyond familial relationships to others. It points out the perversion of sleeping with both a woman (apparently not as her original husband) and her daughter. The mother and the daughter are too near to one another and should not be so intimately linked to the same man.

Verse 18 continues this idea by further prohibiting marrying one woman, and then also marrying her sister. Once again, we do see a patriarch who was on the wrong side of this, Jacob was famously married to both Rachel and her sister, Leah. Of course, that was not his intention but came about by trickery from his father-in-law Laban.

Verse 20 also shows how a neighbor’s wife is too close of a relationship for you to be mixing with. Of course, there is the matter of fornication that already makes such a relationship forbidden, but perhaps this is saying there is even an extra offense if the person you fornicate with is a neighbor. We are also instructed not to lay with a woman during her period or other times of uncleanness.

In all, we are being told that there is such a thing as situational transgression. A woman might in other cases be eligible to woo, but because of a shared element, now becomes too close for anything like that. Before a man and woman become closer than any other, there must first be a sufficient gap between them.